Optical transmission networks allow all-optical transmission between network nodes. Traffic is carried by optical channels, called lambdas, and optical switching technology, such as Wavelength Selective Switches (WSS), allow lambdas to be switched at network nodes, such as Reconfigurable Optical Add/Drop Multiplexers (ROADMs).
A control layer can be added to this kind of network to control the operation of the nodes. A possible control layer is the Transport Network Control (TNC) layer. One issue in this type of network is the relatively long time required to set-up or establish a communication path. This is a particular problem during traffic recovery or restoration operations. When a fault occurs in a network, it is desirable that traffic is transferred to an alternative path as quickly as possible. Delay in setting up the alternative path can result in heavy loss of traffic.
ROADMs provide routing and power leveling functionality in an optical network at a physical level, through the use of wavelength selective switches (WSSs) and optical channel monitors (OCMs). When a new path is created by a ROADM, the ROADM controls the WSS to open, step by step, a new channel wavelength on a specific port, as described below.
FIG. 1 shows a known power-up procedure for establishing a communication path. When a new communication path is to be established at time T1, the power level on the communication path is increased in small steps. During each step a power measurement is taken by an optical channel monitor, to provide feedback on the power level of the new channel. This controlled feedback loop, which takes into account an OCM scan, takes in the order of seconds. Once the new channel is “visible” or within a certain range of a target power level (PTARGET) for the new channel, for example at a power level PFINE, the power level of that channel is managed according to a ROADM power leveling algorithm.
A requirement of a power leveling algorithm, when controlling the power level of a new communication path or channel, is not to affect the traffic on channels that already exist. As a consequence, several small steps are used when setting up a new channel, for example between times T1 and T2, to avoid large changes in the optical power, and in particular to avoid an excessively large power level that might damage a receiver of a subsequent node or component in the communication path. This technique therefore has a delay (T2-T1) associated with powering up the communication path.
Often a communication path can include a chain of ROADMs, each controlling a segment or section of the overall communication path. Each ROADM requires the power level of its section of the new communication path or channel to be created step-by-step in the manner shown in FIG. 1, whereby a WSS opens a path in fixed steps until the channel is detected by the OCM (i.e. between T1 and TF), then each further step is calculated using a smoothing factor (e.g. 0.1 between TF and T2) to avoid any uncontrolled peak of power from being propagated. Once the channel reaches, or is close to, its target power, this information is propagated to the next ROADM in the path which starts the same process. As a consequence, the time taken for the full communication path to become available is dependent on the number of ROADMs involved in the chain.
Existing solutions such as this work well for preventing oscillations in existing channels (which could otherwise affect the performance of existing channels). However, the complete channel provisioning, particularly in a chain of ROADMs, is in the order of tens of seconds or even minutes. While this might not be an issue when creating a new path for new traffic being installed, it becomes disadvantageous in scenarios where fast restoration is desired by a control plane (such as TNC) for a fast provisioning of an optical path, for example because of an issue or fault in another link of the network.